The daughter of the president of Daidouji Toy Company, Sonomi Daidouji, Tomoyo lives a life of wealth and is watched over by a team of female bodyguards. She has access to a variety of prototype technological gadgets from her mother's company, and supplies Sakura and Cerberus with different communication devices throughout the series. She is depicted as being mature, kind-hearted, intelligent, and regularly speaking using more formal verb conjugations and expressions than normally seen in elementary students. She is a talented singer, and is shown performing in various school events. In the anime, her musical ability causes her to be targeted by both the Voice and Song cards.

When she learns of Sakura's new role as a Cardcaptor, she insists that Sakura must wear "special outfits" and begins providing Sakura with the various costumes she wears during her captures as well as accompanying her on missions to record the events with her video camera. In the manga, some scenes give the appearance that Tomoyo has a crush on Sakura's brother Toya; however, according to an interview in the Cardcaptor Sakura Memorial Book, this was an error caused by one of the finishing artists not being aware that CLAMP intended for Tomoyo to have feelings for Sakura. In future volumes, this is corrected by having a scene note that Tomoyo blushed around Toya because he reminded her of Sakura. While Tomoyo is in love with Sakura, she notes that she is happy to be able to be with Sakura and does not require Sakura to love her back. Instead, she encourages Sakura's love of Yukito, and later becomes Syaoran Li's confidante and ally when he falls in love with Sakura. In the English adaptation Cardcaptors, Tomoyo's name is changed to Madison Taylor. The familiar relationship with Sakura is removed and all dialog suggesting that she has romantic feelings for Sakura are edited to be purely platonic in nature.

In the anime adaptation, the character is voiced by Junko Iwao. In Cardcaptors, she is voiced by Maggie Blue O'Hara. In the English dub of Cardcaptor Sakura Movie 2: The Sealed Card, she is voiced by Michelle Ruff.

Tomoyo is an elegant and delicate-looking young girl with porcelain-white skin and long, wavy, grayish-violet hair, which is very similar to that of Sakura's mother, Nadeshiko Kinomoto. It was later revealed that Tomoyo's own mother, Sonomi Daidouji, had made her grow her hair like that so she would look like Nadeshiko. She also has purple eyes, making her and Sakura the only two people to have an eye colour other than brown or gray from her other classmates.

In Cardcaptors, Tomoyo is renamed Madison Taylor. Elements that might have hinted at a romantic attachment to Sakura are removed from the story. Her 'love' for Sakura is completely platonic. Despite the removal of a lovers relationship, Tomoyo still acts as Sakura's best friend and supporter, and loves giving Sakura new battle costumes, and filming Sakura on all her adventures.

The fact Sakura and Tomoyo were second cousins was also erased from the story, mostly due to the network passing over the episode where Sakura and Tomoyo discover the fact they are related. The episode was aired outside America, but their family connection was limited to being merely friends like their mothers were.

Tomoyo is the best friend of the series heroine, Sakura Kinomoto. When she discovers that Sakura has become the Cardcaptor, she becomes Sakura's primary assistant by designing "battle costumes" and filming all of her magical endeavours. Tomoyo faithfully keeps Sakura's new identity secret and often covers for her in times of need.

In the early releases of the manga, one of the artists was not aware that Tomoyo had feelings for Sakura. As a result, some scenes in the beginning chapters show Tomoyo with a slight crush on Sakura's brother Toya. Clamp soon clarified the story with each other, however, and Tomoyo is subsequently seen showing more attention towards Sakura.

Tomoyo is the daughter of the president of Daidouji Toy Company, Sonomi Daidouji. Sonomi is the cousin of Sakura's mother Nadeshiko Kinomoto , which makes Sakura, Toya and Tomoyo second cousins. Tomoyo's father is never shown in the series and is only alluded to once, but it is apparent that he does not live in the Daidouji household. Her western-style house in Tomoeda is quite grand, with several maids serving the household. She is sometimes attended by the Daidouji family's team of female bodyguards, and has access to prototype technology from her mother's company.

Tomoyo is very mature for her age. One of her major character traits is her selflessness towards Sakura. Tomoyo is kind, caring, sweet, highly intelligent, beautiful, meticulous, and very melodic and annoying perfectionist. She enjoys making costumes for Sakura to wear and filming her all the time. She also sings in a club at school. Tomoyo can be slightly over the top at times, but otherwise she is pratically perfect in every way. She always speaks using formal verb conjugations and expressions, giving her a unique air of refinement amongst the cast. When needed, Tomoyo can exhibit considerable cunning and resourcefulness, which was most prominently displayed in the final movie.

Tomoyo aids Sakura's efforts to be in the company of Yukito Tsukishiro on many occasions, and, in the second half of the series, becomes the confidante of Syaoran Li when he finds himself falling in love with Sakura.